Calorimetry as a routine dosimeter at an electron beam processing facility

ABSTRACT

The invention is generally directed to methods of radiation processing and methods for routine dosimetry, such as for the measurement of radiation dosages delivered to items to be processed by the application of radiation. Radiation-induced heat in an irradiated item is measured with a calorimeter and used to determine the dose of radiation received by the item. Resistive elements such as thermistors may be used in calorimeters to measure the change in temperature of an item receiving radiation, such as in electron beam radiation. Calorimeters exposed to irradiation in the same manner as irradiated items provide routine dosimetry for radiation processing.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of and priority from ProvisionalApplication Ser. No. 60/275,556 filed on Mar. 13, 2001, which is herebyincorporated by reference in its entirety.

FIELD OF INVENTION

The invention relates to the fields of dosimetry and calorimetry, inparticular routine dosimetry using calorimetry for the routinemonitoring and control of ionizing radiation processing procedures.

BACKGROUND OF THE INVENTION

Radiation processing, the treatment of items with radiation, plays animportant role in the production of many products. A radiation processis a method or procedure which uses radiation processing, such asradiation sterilization. Radiation may be used in a radiation processfor the sterilization of materials, particularly for medical instrumentsand accessories, for the pasteurization of food products, and formaterial processing, such as for inducing or enhancing thepolymerization of materials or the introduction of dopants or impuritiesinto substantially pure materials. Electron-beam radiation is a commonform of radiation used in radiation processes for sterilization,pasteurization, and for alteration of the properties of materials.

A radiation dosage is an amount of radiation absorbed by an irradiateditem. Dosimetry is the measurement of an amount of radiation.

Measurement, by means of a dosimetry system, and reporting of the amountof radiation absorbed by items to be sterilized or otherwise processedby radiation is an important quality control measure. Reportingrequirements in highly regulated industries, such as those typicallyusing radiation processing, are stringent and often require much effort.Calibration of the dosimetry system, comprising comparison of thedosimetry system measured values with those of national standardradiation sources with different control settings, is typicallyperformed before use of a radiation source, and optionally at othertimes thereafter, to determine whether the dosimetry system is providingaccurate radiation dosage measurements. The calibrated dosimetry systemis used, if necessary, to adjust the radiation source so that itperforms as desired. Quality control measurements may be routinemeasurements taken infrequently or frequently, and often at regularintervals, to monitor the performance of the radiation processing on anon-going basis. Such quality control measurements of radiation deliveryduring radiation processing are termed routine dosimetry measurements,and differ from calibration measurements in that calibrationmeasurements are used to assure the accuracy of the dosimetry system,and to verify that dose measurements obtained are valid, while routinedosimetry measurements track the performance of a radiation processduring use.

Two general categories of dosimeters exist, reference standarddosimeters and routine dosimeters. The objective of the referencestandard dosimeters is to provide a link between national standarddosimetry calibration laboratories and production radiation processingfacilities. The key criteria is control and accuracy of measured dose.Practicality is thus not an issue for reference standard dosimeterssince their use during calibration of routine dosimeters is infrequent.Routine dosimeters, on the other hand, are used for regular qualitycontrol within a radiation processing plant. Practicality is of utmostimportance, combined with reasonable accuracy.

The amount of radiation delivered by an electron beam may be measured ina number of ways. For example, the current induced by the passage ofradiation past capacitor plates was reported as a measure of theradiation beam dosage by Taumann, U.S. Pat. No. 4,427,890, while thecurrent produced in a coaxial sensor placed in an electron beam wasreported to be proportional to an electron beam current intercepted bythe sensor, and so was said to be useful as a measure of the beam dosage(Fiorito et al., U.S. Pat. No. 4,629,975). The current collected by abeam stop was said by Lawrence et al., U.S. Pat. No. 5,661,305, to be auseful measure of the absorbed dose in a product irradiated by anelectron beam. All patents cited herein, both supra and infra, arehereby incorporated by reference in their entirety.

A listing of and a discussion of the advantages and disadvantages ofvarious dosimetry methods may be found in Annex C of “Dosimeters,dosimetry and associated equipment” ANSI/AAMI/ISO 11137-1995 (1995)(referred to herein as “ISO 11137”). For example, calorimetry is listedtherein for use as a reference standard dosimeter, while severalspectrophotometric methods are listed as examples of routine dosimeters.

One method of detecting and measuring radiation is to measure theoptical density of radiation-sensitive films (also known as radiochromicfilms) exposed to radiation. Commonly, calibration is performed byirradiation of unexposed radiation-sensitive film routine dosimeterstogether with controlled reference standard dosimeters from nationalstandards laboratories. The results of such film dosimetry aredetermined by the amount of exposure of the film following irradiation,such as by measuring the optical density of the developed film. Resultsof the reference standard dosimeters are then correlated with theresults of the routine film dosimeters to form calibration curve for thefilm dosimeters. Film dosimetry is performed for routine dosimetry toconfirm that the appropriate amount of radiation is being delivered tothe items to be irradiated, and to correct the exposure if it is foundto be inaccurate. However, there is a delay in obtaining film dosimetryreadings while the film is developed, and film dosimetry is subject toproblems of reliability and consistency due to variation between films,variability of film placement on or within the items being irradiated,variability in the time between irradiation and film development, andthe effect of temperature, humidity and ultraviolet light levels on theexposure characteristics of the film (see, for example, Table C5, ISO11137).

The absorption of radiation heats an object that has been irradiated.Calorimetry, which measures heat, may be used as a method of dosimetryby measuring a temperature change in an irradiated item and correlatingthe radiation dosage absorbed with the temperature change. However, inorder to be accurate, these measurements must be made without allowingany significant loss of heat. Radiation processing with gamma radiation,for example, may take several hours. Electron beam sources typicallyrequire a shielding maze and multiple passes within the maze, requiringan hour or more for radiation processing. Significant amounts of heattransfer might occur during this time, making calorimetry measurementsdifficult and inaccurate under these circumstances. Attempts tocalibrate an electron beam source radiation by placing a calorimeter onan arm that swings into the path of the electron beam require thatradiation processing of other items be stopped, and the systemreconfigured for normal operation before radiation processing is able tobegin again. However, such a system cannot be used for routine dosimetrymeasurements because the calorimeter interrupts the normal functioningof the system.

The variability in the measuring tools presently used in routinedosimetry hinders routine measurement of the performance and reliabilityof radiation sources. However, radiation processing, such as forsterilization, is a heavily audited and critical process in theproduction of medical implements and medical instrumentation, in foodprocessing, and in the processing of many materials. The ability tominimize quality control issues related to routine dosimetry wouldimprove present methods and significantly reduce the risk ofnon-compliance with the strict regulations that are typically applied toradiation processing.

Accordingly, what is required are systems and methods for radiationprocessing and for routine dosimetry that are not affected by sensorlocation, variability in sensing elements, and other such problems, andare capable of being used without interference with the normal operationof radiation processing methods.

SUMMARY OF THE INVENTION

The present invention is directed to a method and system for determininga radiation dose for the quality control of a radiation process, andparticularly to a method and system for routine dosimetry for thesterilization of medical products.

A system embodying features of the invention, such as a system forroutine dosimetry, includes a calorimeter, particularly a thermistorcalorimeter, a radiation source, and a calorimetry control system. Suchsystems are particularly suitable for use in sterilizing medicalproducts.

The calorimeter should have a validated resistance-temperaturecalibration relationship and a validated temperature-dosagerelationship. Irradiation of the calorimeter heats the calorimeter. Thetemperature of a calorimeter after irradiation is preferably measuredbefore significant heat loss has occurred. In routine dosimetry,calorimeter temperature is measured as soon as possible afterirradiation so that only very small amounts of heat are lost.Preferably, the loss of heat from the calorimeter is a linear functionof time after irradiation. In embodiments of the system, the calorimeteris configured to reduce or prevent heat loss, such as by insulation orby maintaining the external temperature near to the temperature of theheated calorimeter.

The radiation source may be a controlled radiation source, and ispreferably a high dose-rate radiation source. The high dose-rateradiation source may be an electron radiation source. Radiation dosesprovided by the radiation sources of the systems of the invention may bebetween about 0.1 kGy to about 100 kGy. In embodiments, the radiationdose is between about 2 kGy to about 70 kGy, and in still furtherembodiments, the radiation dose is between about 3 kGy to about 40 kGy.

The calorimetry control system may include a calorimeter controller,which may monitor and/or direct the performance of the system and itscomponents. The calorimeter controller should be an automaticcalorimeter controller, and preferably should be a computer-controlledautomatic calorimeter controller. In routine dosimetry control theinterval between routine dosimetry measurements is constant, unless thetarget radiation dose has changed, in which case a routine dosimetrymeasurement is taken regardless of the amount of time that has passedsince the previous routine dosimetry measurement. Typically, theinterval between dosage determinations is less than about an hour,preferably less than about a half hour.

The systems may further include a conveyor system for effecting relativemotion between a radiation source and items and calorimeters to beirradiated. The conveyor should be effective to move the calorimeterthrough the path of radiation from the radiation source, preferablywithin a short time. The conveyor may move the calorimeter along ashort, closed-loop route so as to return the calorimeter to and endingposition within a short time. In embodiments, the ending position is oris near to the starting position.

In addition, the systems of the invention may further include a roboticarm having a resistance measuring device able to temporarily contact andobtain a resistance measurement from the thermistor calorimeter.

The present invention further provides methods for routinely determiningand reporting a radiation dose for quality control of a radiationprocess, which is particularly suited to routine dosimetry for thesterilization of medical products. Routine dosimetry methods embodyingfeatures of the invention include measuring an initial calorimetertemperature, irradiating a calorimeter, and measuring a subsequentcalorimeter temperature.

The calorimeter temperature is measured before and after irradiation ofthe calorimeter by the radiation source, and the radiation dose receivedby the calorimeter is determined using the calculated temperaturedifference between the initial temperature and subsequent temperaturemeasurements of the calorimeter, and using the resistance-temperatureand temperature-dose calibration relationships in conjunction with thetemperature differential. Preferably, the irradiation of the calorimeteris performed in the same manner as the irradiation of items, such asmedical products receiving radiation sterilization, is performed. Thedose determination procedure is preferably repeated at an interval, orafter a specified number of items has been processed, or by othercriteria, determined by the calorimeter controller. The determineddosage may then be reported.

The calorimeter controller may be used to determine and/or control theinterval between routine dosimetry measurements. For example, thecalorimeter controller may be used to determine whether the dose fromthe radiation source has been changed, and to maintain the intervalbetween routine dosimetry measurements constant when the radiation dosehas not changed. The calorimeter controller may also be programmed toprompt or initiate a calorimeter measurement whenever the radiation dosefrom the radiation source has changed. In addition, the calorimetercontroller may be employed to determine whether the calorimeter hasvalidated resistance-temperature and temperature-dosage relationships,i.e., is a validated calorimeter, may be programmed to accept data onlyfrom validated calorimeters, and may manage the printing of a processreport.

Thus, a method embodying features of the invention comprises: providinga calorimeter control system and a calorimeter; measuring an initialcalorimeter temperature; irradiating the calorimeter with a dose ofradiation from a radiation source; measuring a subsequent calorimetertemperature before significant heat loss has occurred; determining theradiation dose using a calculated temperature difference between theinitial temperature and subsequent temperature measurements, and usingthe resistance-temperature and the temperature-dosage calibrationrelationships; repeating these procedures at an interval determined bythe calorimeter controller; and reporting the radiation dose. Inembodiments, the calorimeter is a thermistor calorimeter, preferably avalidated thermistor calorimeter having a validatedresistance-temperature calibration relationship and a validatedtemperature-dosage calibration relationship.

Preferably, the calorimeter measurements taken subsequent to irradiationare taken as soon as possible after irradiation so that only smallquantities of heat are lost from the calorimeter, or so that the heatloss will be a nearly linear function of time after irradiation. Tominimize such heat loss, and so to measure the calorimeter temperaturebefore significant heat loss has occurred, the time the calorimetertemperature is taken after irradiation should be a short time, such asless than about an hour, preferably less than about 30 minutes, and morepreferably less than about 15 minutes. Other methods may also be usedfor minimizing heat loss and insuring that calorimeter measurements maybe made before significant heat loss has occurred, such as providinginsulation, raising environmental temperature, or other methods.

In embodiments of the routine dosimetry method, the irradiatingprocedure further comprises movement of a calorimeter by a conveyoralong a route. Such a route may be a short, closed-loop route. Thecalorimeter may be conveyed along a route at a rate effective to returnit to its starting position within a short time, or to an endingposition which may be near to the starting position, within a shorttime. Such a short time may be less then about 30 minutes or,preferably, may be less than about 15 minutes. In embodiments of themethods, calorimeter temperature measurements are taken at locationsdetermined by the conveyor route, typically at locations near to thestarting position and/or ending position along the conveyor. Inembodiments of the method, the time required to convey a calorimeteralong a route between starting and ending positions is substantially thesame as the time interval between initial and subsequent calorimetermeasurements. Such a time interval is preferably a short time.

In one embodiment, the radiation source is a high dose-rate radiationsource, such as an electron radiation source. The radiation dosageoutput capability of the radiation source should be between about 0.1kGy to about 100 kGy, preferably between about 2 kGy to about 70 kGy.For sterilizing medical products such as guidewires, catheters, and thelike for vascular procedures, suitable dosage rates may range from about3 kGy to about 40 kGy.

Determining a radiation dose includes contacting the thermistorcalorimeter with a measuring device and at least temporarily moving thetemperature measuring device while in contact with the thermistorcalorimeter to obtain a calorimetry measurement from the calorimeter.The temperature measurement preferably is a resistance measurement.

The invention also provides a routine dosimetry control method fordetermining at intervals and reporting an acceptable radiation dose in aradiation process comprising a calorimeter that has a maximum lifetimedose. Such embodiments of the routine dosimetry control methods comprisedetermining whether a calorimeter is valid and has received less than amaximum lifetime radiation dose. The calorimeter is determined to be avalidated calorimeter if it is a valid calorimeter and has received lessthan a maximum lifetime radiation dose.

The invention yet further provides a method for radiation processing ofan item, such as a medical product, comprising processing a calorimeterin a radiation process according to a routine dosimetry method of theinvention, and processing an item in the radiation process. Preferably,the calorimeter and the processed items are irradiated in the samemanner, preferably by the same radiation source.

In addition to the particular embodiments listed above, it will beunderstood that other combinations and arrangements of the componentsand procedures of the systems and methods described may be used in thepractice of the invention. All such combinations and arrangements arewithin the scope of the present invention.

The routine dosimetry systems and methods of the present inventionprovide benefits that include reduced labor requirements, reducedenvironmental requirements, and increased consistency and quality ofdosimetry results for both processing dosimeters and investigatingdosimetry issues. Other dosimetry methods commonly used at present, suchas film or other spectrophotometric methods, have disadvantages such asrequiring the calibration of optical equipment and the maintenance ofspecial environments.

Practice of the methods of the present invention are effective to reducethe labor needed to perform a radiation process; for example, anautomatic calorimeter controller eliminates the need for a person toread or process dosimeters, improves reliability and repeatability ofthe processes, thereby reducing stoppages and reducing wastage.Automatic control of a radiation processing system allows for theautomatic collection and recording of data for process reports, reducingthe costs of compliance with regulatory requirements. Thus, theradiation processing and routine dosimetry as performed in the practicethe present invention can be accomplished with fewer resources, with farsuperior quality and with far less compliance risk than prior methods.

These and other advantages of the invention will become more apparentfrom the following detailed description of the invention and theaccompanying exemplary drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram illustrating a routine dosimetry control methodembodying features of the invention.

FIG. 2 is a flow diagram illustrating a method of radiation processingcomprising a routine dosimetry control method in a radiationsterilization process embodying features of the invention.

FIG. 3 is a plan view of a radiation processing system embodyingfeatures of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides improvements in radiation processing,comprising improvements in methods of measuring radiation in methods andsystems for radiation processing. The inventors here disclose novelmethods and systems for the use of calorimetry for radiation processingsuch as radiation sterilization and routine dosimetry in methods andsystems for sterilization. The novel methods may be particularly usefulas systems and methods for routine dosimetry for sterilizing medicalproducts. The novel methods for routine dosimetry comprise the routinemonitoring of the performance of ionizing radiation processingfacilities using calorimetry. A preferred embodiment of the method is amethod of utilizing calorimetry for routine dosimetry in a radiationprocess. A radiation process may comprise, for example, a process forradiation sterilization of an item. It should be noted that the terms“process” and “method” as used herein are synonymous.

Thus, the invention provides methods and systems for routine calorimetrymeasurements for routine dosimetry for quality control of a radiationprocess. In preferred embodiments, the system has a short, closed-looproute in the shape of a simple oval race track-like processing conveyorand shielding design that accommodates rapid transport between the loadand unload areas. In this manner, irradiated items may be loaded ontothe conveyor, receive radiation and arrive at the unload area in a shorttime. In preferred embodiments, the thermistor calorimeter is providedwith adiabatic insulation to retain heat evolved during irradiation.Thus, the systems and methods of the invention make insignificant theloss of the heat generated by the irradiation during these short timeperiods, so that, where the irradiated item is a calorimeter for routinedosimetry, calorimetry measurements are reliably correlated to theradiation dosage, and corrections for the short periods of time betweenirradiation and temperature measurement are minimal and easy to predictaccurately.

The system and methods comprise a calorimetry controller whichdetermines whether a calorimeter is valid. A valid calorimeter has avalid resistance-temperature relationship and a valid temperature-doserelationship. Valid resistance-temperature and temperature-doserelationships comprise relationships in which a dose determined usingthe resistance-temperature and temperature-dose relationships and a dosetraceable to a dose determined by a national calibration laboratory dosemeasurement match to within a specified degree. In embodiments of theroutine dosimetry control method, the specified degree of match betweena dose traceable to a dose determined by a national calibrationlaboratory dose measurement and the dose determined using theresistance-temperature and temperature-dose relationships is less thanabout 15%; in yet further embodiments, the specified degree of match isless than about 5%.

A database of validated calorimeters may be kept, calorimetersidentified, as by a bar code or other label or identification, and byscanning the bar code or otherwise identifying a calorimeter, thecalorimeter controller may determine whether the calorimeter is valid.Criteria for validity include resistance-temperature andtemperature-dose relationships that are within specified ranges, and acumulative lifetime level of radiation exposure that is less than amaximum lifetime dose. Calibration relationships may be determined bycomparing calorimeter dose measurements to dose measurements made underthe same conditions using national calibration laboratory dosestandards. The National Institute of Standards and Technology (NIST) andthe National Products Laboratory (NPL) in the United Kingdom providesuch calibration standards.

Calorimetry is an inherently simple, accurate and precise measurement ofthe change in temperature of an item. Absorption of radiation by anirradiated item results in heating of the item that is correlated to thedose of radiation received by the item. It is possible to measure theheat generated by irradiation, and so to obtain a measure of the amountof radiation received by the irradiated item. In particular, the heatingof an item by electron beam irradiation may be measured by a calorimeterto provide a measurement of the dose absorbed by an irradiated item.Thus, calorimetry is suitable for measuring the radiation dose absorbedby an item (see, e.g., Table C1 of ISO 11137, and Table 1, “Standardguide for selection and calibration of dosimetry systems for radiationprocessing” American Society for Testing and Materials, 1994 publicationASTM E 1261-1994 “ASTM 1261”).

Such radiation-induced heating is a reliable measure of the amount ofradiation arriving at the item, being directly caused by the interactionbetween the impacting radiation and the irradiated object. For thisreason, calorimetry is not subject to the kinds of errors that othermethods of radiation measurement are subject to, such as variability insensing elements or in the location of a sensor with relation to theobject. In addition, unlike other methods of measuring radiationdosages, variables in the testing environment such as humidity andultraviolet light level do not affect calorimeters. These benefits arethe reason for calorimetry being used as a reference standard dosimetrymethod (see, e.g., Table 1, ASTM 1261 and C1 of ISO 11137).

Reference is made to FIG. 1, wherein an embodiment of the invention,comprising a routine dosimetry control method, is shown in schematicform. It will be appreciated that the method disclosed in FIG. 1comprises one of many embodiments of the invention, and that otherembodiments may vary, so that, for example, other procedures may also beincluded or not every procedure illustrated in the process disclosed inFIG. 1 need be performed in the practice of the invention.

The method illustrated in FIG. 1 is a routine dosimetry control methodcomprising a method for measuring a radiation dose from a radiationsource, such as a high dose-rate radiation source. A high dose-rateradiation source typically provides radiation at a dose rate of at least10 Gy/s, typically at dose rates of about 100 to about 1000 Gy/s, moretypically at dose rates of about 250 to about 1000 Gy/s (where “Gy” is aGray, a radiation dose equivalent to the absorption of 1 joule perkilogram). The radiation source has a target radiation dose rate that isthe dose rate desired to be delivered by the radiation source. Thetarget dose rate is typically specified by beam parameters used by aradiation beam controller, such as an electron beam controller. A targetdose in a radiation process is the dose that is desired to be deliveredto an irradiated item by the beam source. Routine dosimetry measures andrecords the radiation doses actually delivered by the radiation sourceproviding the routine measurements required in quality control of aradiation process.

The method illustrated in FIG. 1 comprises the procedures of irradiatinga calorimeter, and measuring heat with the calorimeter effective toprovide a measurement of heat evolved from the irradiating procedure. Inthe practice of this method with a thermistor calorimeter, the procedureof measuring heat comprises taking a resistance measurement andcalculating a temperature difference using the calorimeter'sResistance-Temperature (R-T) calibration relationship (a relationship isalso termed a curve, so, e.g., this is also termed the R-T calibrationcurve) to provide a measurement of the heat evolved from the irradiatingprocedure. By means of the calorimeter's Temperature-Dose (T-D)calibration relationship or curve, the temperature difference and themeasurement of the evolved heat derived from it can be used to determinethe amount of radiation received by the calorimeter.

The method of FIG. 1 is illustrated by means of a flow chart, in whichthe procedures of the process are represented by a square, a diamond, ora square with one rounded side. Squares represent procedures of theprocess where the process proceeds directly to the next procedure uponcompletion of the indicated action; diamonds indicate branch points fromwhich lead alternative paths, so that a decision must be made todetermine which of the alternate paths to follow; and a rounded squareindicates a halt in the process, where the process waits for a time oronly continues to the next procedure upon fulfillment of a criterion.Arrows indicate the temporal direction of the process, with proceduresat the arrow head being subsequent to procedures at the tail of thearrow. Thus, in the processes illustrated in FIG. 1 and in FIG. 2, thedirection of an arrow, from tail to arrow head, indicates the sequenceof procedures to be taken in the processes illustrated.

The embodiment of the method illustrated in FIG. 1 begins with procedure1 termed “Calorimeter Process Start” signifying the initiation of theprocess. Procedure 2, “Prompt Calorimeter,” is a procedure in which thecontrol system prompts the operator that, based on the process criteria,the time has arrived for another routine calorimeter measurement and acalorimeter should be entered into the system (e.g., onto the conveyorsystem) for processing. In an automatic or partly automatic process,where an operator need not be involved at this procedure, the promptwould consist of an automatic signal, or to automatically beginning aroutine dosimetry measurement. Examples of such process criteria thatcould trigger the initiation of another routine dosimetry measurementinclude the passage of a pre-determined interval of time, and a changein the target radiation dose-rate of the radiation source.

Procedure 3, “Identify Calorimeter” comprises a procedure where theindividual calorimeter is identified, as, for example, by noting aserial number, or reading a bar code. (The calorimeter may have alreadybeen identified prior to procedure 2 if the process had proceeded toprocedure 2 from procedure 7.) Procedure 4, “Calorimeter has Valid Cal.Curve?” referring to a calibration curve for the particular calorimeter,comprises a procedure where the identification of the calorimeter isused to check whether that particular calorimeter has been properlycalibrated, and whether the calibration of the calorimeter is stillvalid. The calibration relationship involved at this procedure istypically with regard to the R-T calibration curve and the T-Dcalibration curve. Thus, a calorimeter passing this procedure has beenvalidated as to its calibration curve. Such a check may be performed byinspection of written calorimeter calibration records, bycomputer-assisted inspection of digital records, by performance of acalibration, or by other means known in the art. Procedures forcalibration of the calorimeter will differ between different types ofcalorimeters, but are well known to those of ordinary skill in the art.For example, calorimetric methods for dosimetry may be found in “Use ofCalorimetric Dosimetry Systems for Electron Beam Dose Measurements andDosimeter Calibrations” (Annual Book of ASTM Standards, publication E1631-96, American Society for Testing and Materials, West Conshohocken,Pa. 19428 (1999) “ASTM 1631”). In general a calorimeter calibrationprocedure comprises a series of measurements taken at different knownradiation doses, so that the output of the calorimeter as a function ofradiation dose is thereby determined. It will be understood by those ofordinary skill in the art that the criteria for validity of thecalibration may vary, and may depend on the type, sensitivity, andstability of the calorimeter, any regulatory requirements that mayapply, and other factors known to those of ordinary skill in the art.Procedure 4 is a branch point in the process, where the next procedureto be taken is determined by the decision made during procedure 4. Inthe instance where the calorimeter does not have a valid calibrationcurve, the next procedure in the process after procedure 4 is procedure7, “Record Cal. Invalid.” If, however, the calorimeter does have a validcalibration curve, the next procedure in the process is procedure 5.

Procedure 5, “Calorimeter<max dose?” again uses the identification ofthe calorimeter from procedure 3 to allow inspection of calorimeterrecords to determine whether the cumulative radiation received by aparticular calorimeter during its operational lifetime is below or is ator has exceeded its allowed cumulative radiation dose. Thus, acalorimeter passing this procedure has been validated as to its lifetimemaximum dose. The actual value of a maximum dosage for a calorimeterwill vary, and may depend upon the type of calorimeter (includingdepending on the materials used, the design and the method ofmanufacture of the calorimeter), the sensitivity of measurement that isrequired, regulatory requirements that may apply, and other factorsknown to those of ordinary skill in the art. Typically, a maximumlifetime dose will be in the range of about 1 to about 5 megaGray (MGy)of radiation. Information regarding such factors may be found in ASTM1631.

Thus, procedure 5 is also a branch point; if the calorimeter has reachedits maximum dose, procedure 5 leads to procedure 6; if, however, thecumulative radiation dose absorbed by the calorimeter is below themaximum dose, then the process proceeds to procedure 8. Procedure 6,“Calibration Due” indicates that, where a calorimeter has reached itsmaximum dose, the calorimeter must be re-calibrated (to verify itscontinued accurate performance) before using it to monitor a radiationdose in a sterilization procedure, or must be replaced by anothercalorimeter that has not received an excessive amount of radiation.Following procedure 6, if the maximum dose has been exceeded, orfollowing procedure 4 if the calibration curve was not valid for anyreason, the process goes to procedure 7, where the calorimeter is markedor listed in a record book, or listed in an electronic record such as adatabase, or otherwise identified as not available for further use in acalorimeter process. After procedure 7, the process then returns toprocedure 2. In this case, the calorimeter arriving at procedure 2 isone in need of re-calibration, and procedure 2 then comprises acalibration procedure, including appropriate updating of recordsregarding the calorimeter. From this point, the process continues onagain to procedure 3. If, however, the calorimeter fails therecalibration at procedure 2, or is otherwise determined to be unusableat procedure 2, instead of proceeding to procedure 3 the processproceeds to procedure 19 (“End if not calibratable”) with the rejectionof the calorimeter and ending of the procedure. The process may be begunagain at procedure 1 with a different calorimeter.

Where the calorimeter has a valid calibration curve and has not exceededits maximum dose, the process proceeds from procedure 5 to procedure 8.Calorimeters passing procedures 4 and 5 may be termed “validatedcalorimeters.” Procedure 8, “Initial Calorimeter Measurement,” is acalorimetry measurement taken before irradiation of the calorimeter inorder to provide a baseline measurement. At procedure 9, “Record InitialMeasurement,” the results of the initial calorimetry measurement made atprocedure 8 are entered into the recording system, e.g. by hand into alog-book, or by keyboard entry into a computerized record system, or thelike. It will be understood by one of ordinary skill in the art thatthere are many suitable methods for recording the results of acalorimetry measurement, and that any such method may be used in thepractice of the invention.

At procedure 10, “Calorimeter Processed Through Beam,” the calorimeteris irradiated as it is processed through a radiation beam, such as anelectron beam. After irradiation, another calorimeter measurement istaken at procedure 11, “Calorimeter Measurement Following Irradiation,”and the results of the measurement are recorded at procedure 12, “RecordMeasurement After Irradiation.”

It is critical that only a short time intervene between the irradiationof the calorimeter at procedure 10 and the calorimetry measurement atprocedure 11. During such a short time, the heat loss from thecalorimeter is negligible or very small, so that the measurement of theheat evolved during the irradiation procedure is, or is very nearly, anadiabatic measurement. A short time may comprise less than about anhour, or preferably less than about half an hour. In more preferredembodiments, the time between irradiation of the calorimeter and thecalorimeter measurement is less than about 15 minutes. In addition, bycalibrating the temperature change properties of a particular dosimeter,it is possible to account for the temperature change that may occurduring the time between irradiation and the routine dosimetrymeasurement and so to improve the accuracy of the process.

Providing a short route of travel for the calorimeter as it is conveyedthrough the path of the radiation beam is effective to provide thatthere is only a short time between calorimeter irradiation (procedure10) and calorimetry measurement (procedure 11). For example, a conveyorroute comprising a short oval route is effective to provide such a shorttime. Alternatively, calorimetry measurements may be made with measuringinstruments or measuring tools that are able to contact the calorimeterat points near to the radiation beam on the conveyor route. For example,a robotic arm with a means for measuring calorimeter resistance (such asa probe with an electrical contact connected to a resistance meter)capable of contacting the calorimeter by swinging over the conveyorroute and measuring calorimeter resistance as the calorimeter emergesfrom the radiation beam path is effective to provide that there only bea short time between the irradiation procedure and the measuringprocedure.

A calorimeter controller may take into account the change in temperaturethat occurs in the time between irradiation of the calorimeter atprocedure 10 and the calorimetry measurement at procedure 11.Measurements have shown that this change in temperature is linear for upto an hour after irradiation. In embodiments of the invention, thecalorimeter controller uses the known relationship between temperatureand time after irradiation for an identified calorimeter to calculatethe temperature at the time of irradiation from the measured temperatureat a known time after irradiation, and thereby obtain a calculated dosethat is more accurate than one not taking this change in temperatureover time into account.

Following these measurements, a branch point is reached at procedure 13“Dose Measurement Acceptable?” and a decision is made based on theamount of radiation received by the dosimeter. The amount of radiationthat constitutes an acceptable dose is a parameter of the process thatwill be determined before the process begins, and will vary with theparticulars of the items to be irradiated, regulatory requirements, andother particulars. The dosimeter is a calorimeter, and the radiationdose absorbed by the calorimeter may be calculated by subtracting thetemperature measured at procedure 8 from the temperature measured atprocedure 11 to calculate the change in temperature induced by theirradiation procedure and this change in temperature is used tocalculate the amount of radiation by means of the calorimeter'scalibration curve. Where the calorimeter is a thermistor calorimeter,the temperature measurement comprises a resistance measurement. If theradiation dose is acceptable, then the radiation dose is entered intothe records at procedure 14 “Note Acceptable Dose” by any suitablemethod, as discussed above, and then the process may continue or end atprocedure 15, labeled “Return or End.” Where the process continues, theprocedure following procedure 15 is procedure 1; where the process ends,procedure 15 is the last procedure in the process.

If the radiation dose absorbed at procedure 10 is not acceptable, thenthe procedure 16, “Notify Operator” will follow procedure 13, where theoperator of the process will be notified so that corrective action maybe taken to bring the system and process into conformance with itsdesired performance. Following procedure 16 is procedure 17, “NoteUnacceptable Dose,” which provides that the records of the processinclude a notice that the radiation dose was not acceptable, followed byprocedure 18, “Stop Further Processing Until Issue Resolved” whichinsures that no further poorly-processed items result from the process,and that the problem is dealt with expeditiously. After procedure 18 theprocedure proceeds to procedure 15 “Return or End” where the nextprocedure is procedure 1 if the problem that caused the branch fromprocedure 13 to procedure 16 has been rectified, or the process ends ifthere has been no rectification of the problem.

The methods of the invention further provide an irradiation method forirradiating an item comprising the procedures of irradiating acalorimeter, measuring heat with the calorimeter effective to provide ameasurement of heat evolved from said irradiation procedure, calculatinga change in temperature from said measurement, calculating a radiationdose using the calculated change in temperature, and irradiating anitem.

The method of the invention illustrated in FIG. 2 is a method ofradiation processing. This method, shown in schematic form, comprises anembodiment of the invention wherein the radiation processing comprisesradiation sterilization of items in need of sterilization. It will beappreciated that the method disclosed in FIG. 2 is one embodiment of theinvention, that other procedures may also be included, and that notevery procedure illustrated in the process disclosed in FIG. 2 need beperformed in the practice of the invention.

A calorimeter process comprises a process wherein a calorimeter is usedto measure a radiation dose as illustrated in FIG. 1. The calorimeterprocess referred to in FIG. 2, by, e.g., the boxes 26, 32, 34, and 36labeled “Calorimeter Process,” may be a calorimeter process asillustrated in FIG. 1, procedure 13, or other such routine dosimetrycontrol process of the invention disclosed herein where a calorimeter isused to measure a radiation dose.

The sterilization method of FIG. 2 is initiated at procedure 21,“Start,” and proceeds to procedure 22 “Item Logged In” where identifyingindicia about items subject to the process are recorded. It will beunderstood that such recordation can be performed by hand, mechanically,as by, e.g., a printer, electronically, as by a bar-code reader ormagnetic device, or by other suitable means known in the art, eitherautomatically or under the control of a human operator. At procedure 23,“Item in presterile staging,” an item to be sterilized is readied forsorting and for loading onto a conveyor for transport during asterilization process. Following procedure 23, the item proceeds toprocedure 24, “Item sorted by processing parameters,” where the item maybe grouped with other items which are desired to receive the sametreatment. At the next procedure, procedure 25, labeled “ParametersEntered,” the proper treatment parameters are applied to the electronbeam source and conveyor mechanism to provide the desired radiationtreatment. The entry of parameters may be done manually by an operator,manually or automatically by means of a computer, by a scanner andbar-code system, or by other suitable means known in the art. Then, atprocedure 26, labeled “Calorimeter Process,” in a routine dosimetrycontrol process of the invention, e.g., a process like the oneillustrated in FIG. 1, a calorimeter is subjected to the radiationtreatment that is also to be given to the items in need of radiationtreatment. As will be appreciated from inspection of FIG. 1, if thecalorimeter process of procedure 26 results in an unacceptable dose, asdetermined by, for example, a procedure as in procedure 13 of FIG. 1,then the process of FIG. 2 halts until the issue has been resolved.Appropriate action may be taken in order to achieve a successfulcalorimeter process. For example, where the performance of a calorimeterprocess is initially unacceptable, achievement of a successfulcalorimeter process may require readjustment of the radiation source,e.g., readjustment of an electron beam. Indeed, after procedures 1-5 ofFIG. 1, it is anticipated that all calorimeter measurements taken atprocedure 13 of FIG. 1 (corresponding to procedures 26, 32, 34 and 36 ofFIG. 2) are valid measurements indicating quality concerns related tothe radiation source. Only in the scenario of a malfunction duringprocedures 1-5 of FIG. 1 would an unacceptable calorimeter at procedure13 of FIG. 1 (procedures 26, 32, 34 and 36 of FIG. 2) lead to thepotential for the recalibration of a calorimeter that had an invalidcalibration curve, or the replacement of an unacceptable calorimeter(e.g., one that had received an excessive amount of radiation) withanother calorimeter that is acceptable.

Upon successful completion of a calorimeter process 26 in FIG. 2, itemsto be processed receive their radiation processing, as indicated byprocedure 27 “Lot processed” where the lot may comprise one or aplurality of items desired to receive radiation processing.

Following the radiation treatment of the items in procedure 27, it isdetermined whether there remain more items to process, as indicated bythe diamond 28, labeled “More items to process?” In the event that thereare no more items to be processed, then a further calorimeter process,procedure 36, is performed. Following this, at procedure 37, a record ofthe process is printed, as may be called, for example, a process reportor an E-Beam History Report (EHR), and the sterilization method ends atprocedure 38. The procedure of generating a process report can beperformed by hand, mechanically, as by, e.g., a printer, electronically,as by a computer, or by other suitable means known in the art, eitherautomatically or under the control of a human operator. The processreport may be printed, or displayed and stored electronically ormagnetically. The calorimeter process 36 provides for verification ofthe radiation parameters after the items have received their radiationtreatment, so that the radiation treatment is measured before (procedure26) and after (procedure 36) the items to be sterilized are submitted tothe radiation sterilization procedure.

In the event that more items remain to be treated after procedure 27,then the result of the determination of procedure 28 will be that thereare more items to be processed; in that case, the process proceeds toprocedure 29, “Load next lot,” in which procedure the processed itemsare unloaded and the next lot of items to receive radiationsterilization treatment are loaded onto the conveyor. Then, at procedure30, labeled “Next lot same parameters?,” it is determined whether theradiation treatment parameters for this next lot of items is the same asthe radiation treatment parameters for the previous lot. The properparameters for radiation sterilization and for other radiationprocessing procedures will vary with the items to be sterilized orprocessed, the applicable regulatory requirements, and other factorsknown to one of ordinary skill in the art. If the parameters for thenext lot are the same as for the preceding lot, then no change in theradiation treatment parameters is necessary, and the process proceeds toprocedure 31, “>1 hour since last calorimeter?”, where it is determinedwhether the time since the last calorimeter process is greater than onehour or not. If a calorimeter process was performed within the lasthour, no additional calorimeter process will be performed at this time,but instead the process returns to procedure 27, “Lot Processed” and thenext lot of items receives radiation treatment. If, however, more thanone hour has elapsed since the most recent calorimeter process, whetherat procedure 26, 32, 34, or 36, then another calorimeter process isperformed, at procedure 32, allowing verification that the radiationtreatment is still performing within its specified parameters. Followingprocedure 32, a record of the calorimeter process is printed atprocedure 33, labeled “Print Record.” The record produced at procedure33 provides a record of the monitoring procedure 32. After procedure 33,the next procedure is procedure 27 where the next lot of items receivetheir radiation treatment. It will be understood that requiring routinedosimetry if more than one hour has elapsed since the previous routinedosimetry control process (e.g., a calorimeter process as illustrated inFIG. 1) is one method of the invention, but that providing for routinedosimetry control processes at other time periods, greater than onehour, or less than one hour, or at irregular time periods, are all alsowithin the teaching of the present invention. Thus, in a preferredembodiment, items may be processed for a time, and then a calibrationprocedure, as in a routine dosimetry control process of the invention,may be performed before processing further items. Such repeated routinedosimetry control processes as part of a radiation process, such as amethod for sterilization, provide for routine monitoring andverification of radiation process procedures, and so aid in theirperformance and allow for their ready verification and documentation. Asmentioned above with regard to procedure 26, and as is the case for eachcalorimeter process 26, 32, 34 and 36, in the event that the calorimeterprocess 32 determines that the calorimeter has exceeded its maximumdose, or has an invalid calibration curve, or is otherwise unfit for usefor any reason, a different calorimeter may be put into place andsubjected to the calorimeter process, or, the process may halt until itmay be properly performed.

If, at procedure 30, it is determined that the next lot is not toreceive the same parameters as the previous lot, then from procedure 30the process proceeds to the calorimeter process at procedure 34,providing a calorimeter process that monitors system performance aftercompletion of the radiation treatment analogous to the calorimeterprocess at procedure 32. Thus, for example, when the radiation beamparameters are changed to provide for a different target radiation dose,a routine dosimetry measurement will be made regardless of the intervalof time elapsed since the previous routine dosimetry measurement. Theresults of the calorimeter process 34 are recorded, e.g., saved in aprocess report such as an E-beam History Report, at procedure 35,labeled “Print Record.” It will be understood that display of anelectronically or magnetically generated or stored record is included inthe term “print.”

The process returns then to procedure 25, where the new parameters areapplied, and the performance of the system with the new parameters ismonitored in the calorimeter process at procedure 26. The next lot ofitems then receives its radiation treatment at procedure 27 as theprocess continues as described above.

The radiation sterilization method illustrated in FIG. 2 is an exampleof one particular type of radiation process suitable for the practice ofthe invention. Thus, the sterilization method illustrated in FIG. 2provides an example of an embodiment of the invention wherein theroutine dosimetry control methods of the invention, such as illustratedin FIG. 1, are used to improve quality control in a method of radiationprocessing of items. It will be understood that methods may vary in thepractice of the invention, and that the order of the procedures, theintervals between calorimeter processes, whether or not, and the method,frequency, and types of records maintained regarding the process, andother particulars of the method may differ while still being within thescope of the present invention.

Reference is also made to FIG. 3, which illustrates a radiationprocessing system 40 embodying features of the invention. Radiationprocessing system 40 comprises a radiation source 42 effective to emit aradiation beam 44 along a radiation beam path 46 that crosses conveyor48 that follows a short oval route in this illustration.

The system further comprises a conveyor 48, here shown as following ashort oval course, suitable for carrying an item 50 and a calorimeter52. The calorimeter 52 is suitable for use as a dosimeter. The item andthe calorimeter may be loaded onto the conveyor 48 at the load area 54and may be unloaded, for example, after processing, at area 56. Conveyor48 is effective to carry item 50 and calorimeter 52 through radiationbeam 44 across the radiation beam path 46. Radiation beam 44 thuscontacts item 50 and calorimeter 52, effective to irradiate the item 50and the calorimeter 52.

A calorimeter measurement may be taken at measuring station 58,information being carried to controller 60 by communication line 62. Theshort oval route shown is effective to provide that a calorimetermeasurement may be made within a short time after irradiation. Thecontroller may comprise a signaling mechanism effective to signalwhether a calorimeter measurement has been made, and whether themeasurement is within prescribed specifications. Such a signalingmechanism may comprise a mechanical signal mechanism (such as a flag orother indicator), an audible signal (such as a bell or other soundingdevice), a visual signal (such as a light) or, preferably, an electronicsignal (such as an analog or digital signal capable of being read and/orrecorded by a computer system). In embodiments of the invention,measuring station 58 comprises a location where a calorimetrymeasurement is taken. Measuring station 58 may comprise a location wherean operator performs a calorimeter measurement, e.g. by using aresistance meter to measure calorimeter resistance. Alternatively,measuring station 58 comprises an automatic measuring system, such as,for example, a robotic arm effective to move near to or onto theconveyor route and to contact a calorimeter to make a calorimetrymeasurement, as by measuring calorimeter resistance. In a most preferredembodiment, radiation processing system 40 is a radiation sterilizationsystem. In preferred embodiments, the measurement is able to beautomatically recorded or entered into an electronic record of theprocess, such as into a computer system configured to record themeasurements.

In preferred embodiments of the system, radiation source 42 is a highdose-rate radiation source such as an electron beam source. Irradiationcauses heat to evolve in the item and in the calorimeter, the amount ofheat evolution being proportional to the dose of the irradiationprovided by radiation beam 44. A calorimetry measurement by calorimeter52, such as one made at measurement station 58, is effective to allowcalculation of a temperature change caused by irradiation; such acalculated value for a temperature change is effective to allowcalculation of a radiation dosage received by the calorimeter 52, fromradiation beam 44, that would produce such a temperature change. Thus,the radiation processing system illustrated in FIG. 3, which is mostpreferably a radiation sterilization system, is effective to provide aroutine dosimetry measurement for monitoring a radiation process and foruse in a sterilization method. In preferred embodiments, controller 60comprises a device configured to perform the procedures of a routinedosimetry control method, such as a routine dosimetry control method asillustrated in FIG. 1.

Calorimeter controller 60 is in communication with radiation source 42,which may be a controlled radiation source, via communication line 64.Communication line 64 is effective to allow calorimeter controller 60 tomonitor radiation source 42, so that, for example, calorimetercontroller 60 is able detect when radiation beam parameters change (beamparameters include, e.g., the target radiation dose, that is, thedesired dose of radiation to be delivered). Calorimeter controller 60 isable to detect radiation beam parameters via communication line 64 forinclusion in the process reports generated by the routine dosimetrycontrol methods and routine dosimetry methods of the invention.

Calorimeters measure temperature change in a variety of ways, any ofwhich is suitable for use in the present invention. Thermistorcalorimeters utilize the well-known thermal-response properties ofmaterials whose electrical resistance changes with changes intemperature.

In the most preferred embodiments of the invention, the calorimeter is athermistor calorimeter, and the procedure of measuring heat withcalorimeter comprises a resistance measurement. Thus, as illustrated inFIG. 3, in a most preferred embodiment of the invention, the calorimeter52 is a thermistor calorimeter and heat is measured by calorimeter 52 bymeans of a resistance measurement. In most preferred embodiments, thecontroller 60 comprises a device configured to perform the procedures ofa routine dosimetry control process.

Temperature values may be calculated from thermistor resistancemeasurements by use of empirical mathematical relationships betweenthermistor temperature and thermistor resistance. The change inresistance with temperature may be a linear function of temperature, asis common with thermistors of the PTC type (where increasing temperatureincreases thermistor resistance), or it may be a non-linear function, asis common with thermistors of the NTC type (where increasing temperatureleads to decreasing thermistor resistance). The relationship betweenresistance and temperature of a particular thermistor type is known inthe art, as may be found, for example, in tables in the CRC Catalog ofChemistry and Physics and the CRC Handbook of Material Science (ChemicalRubber Company, Cleveland, Ohio) and other standard references.

In addition, the temperature response characteristics of a type ofthermistor or of an individual thermistor may be measured in acalibration procedure carried out by the manufacturer, supplier, or userof the thermistor. Such calibration procedures comprise heating orcooling the thermistor to known temperatures, and measuring thethermistor resistance at known temperatures. Temperature may be measuredby methods known in the art, for example, by a thermometer, andresistance may be measured by methods known in the art, for example, byan ohmmeter. Resistance-Temperature calibration curves (R-T calibrationcurves), which are graphs of thermistor resistance versus temperature,are commonly made from such calibration procedures, and are typicallysupplied with each thermistor provided by commercial suppliers. (Notethat the R-T calibration curves are to be distinguished from thepreviously-mentioned Temperature-Dose (T-D) calibration curves.)Preferred thermistor temperature readings are obtained when the R-Tcalibration curve is made from calibration measurements that bracket thedesired operating temperature range. Thus, where a thermistor is to beused for temperature measurements near 50° C., thermistor calibrationmeasurements should be made at temperatures both below and above 50° C.as well as near to 50° C., e.g., at 25° C., at 50° C., and at 75° C.

The relationship between thermistor temperature and thermistorresistance may be found by fitting a polynomial to resistancemeasurements obtained at various temperatures. The relationship ofthermistor resistance to temperature is discussed in detail in “Theexactness of fit of resistance-temperature data of thermistors withthird degree polynomials” by M. Sapoff et al., in Temperature ItsMeasurement and Control in Science and Industry Vol. 5, pages 881-882,ed. J. Schooley, American Institute of Physics, New York, N.Y. (1982).In general, the relationship between temperature and thermistorresistance may be given by polynomials of degree N as:

1/T=C ₀ +C ₁(ln R)+C ₂(ln R)² +C ₃(ln R)³ + . . . +C _(N)(lnR)^(N)  Equation 1

ln R=A ₁ +A ₂ /T+C ₃ /T ² +A ₃ /T ³ + . . . +A _(N) /T ^(N)  Equation 2

In most cases, N need be no greater than 3. In addition, it is often thecase that one may dispense with the second-degree term in suchpolynomials without greatly affecting the goodness-of-fit of thetheoretical curve to the empirical data. For this reason, as discussedin Sapoff et al., a commonly used empirical mathematical relationshipbetween thermistor temperature and thermistor resistance is theSteinhart-Hart equation, which generally lacks a second-degree term. TheSteinhart-Hart equation, or variations of it, may be used to create acalibration relationship and calibration curve for a thermistor wherethe resistance of the thermistor is known at three differenttemperatures. A form of the Steinhart-Hart equation relating temperatureto resistance that is useful for such calibration relationships is

1/T=C ₁ +C ₂×(ln R)+C ₃×(ln R)³  Equation 3

where T is temperature in degrees Kelvin, C₁, C₂, C₃ are constants to bedetermined by the calibration relationship, and R is the measuredresistance at temperature T. If a lesser degree of accuracy is required,then the above equation may be used without the last term, C₃×(ln R)³ tomake for a simpler calculation. This reduced equation with only twoconstants is

1/T=C ₄ +C ₅×(ln R).  Equation 4

Resistance measurements made at different known temperatures may be usedin the above equation to calculate the constants. Since there are threeunknown constants in the Steinhart-Hart equation, Equation 1, threeresistance measurements at three known temperatures are sufficient toprovide for three equations to solve for the three unknowns C₁, C₂, andC₃. Where lesser accuracy is required, and the third term is dropped asin Equation 2, two measurements to provide two equations will besufficient to solve for the two unknowns C₄ and C₅ of the reducedequation. Sapoff et al. provide more detail regarding the use of theSteinhart-Hart equation. In addition, monographs and computer programsare available to aid in such calculations. For example, the articlesentitled “Negative Co-efficient Thermistors,” Articles I-IV, fromSensors, Vol. 14, by G. Lavenuta, 1997 (Article I, Vol. 14(5) pages 46,48, 50, 52, 54-55; Article II, Vol. 14(6) pages 4749; Article III, Vol.14(7) pages 17-18, 20; Article IV, Vol. 14(8) pages 54-56) providegraphs and information regarding thermistor properties. BASIC programsfor the solution of these equations are available from ILX Lightwave(Bozeman, Mont.) in their application note #4, which may be found athttp://www.ilxlightwave.com/library/appnotes/04/no4i.htm or otherwiseobtained from ILX Lightwave.

As discussed in Sapoff et al., such polynomials may be valid over arelevant temperature range. Thus, from equations 1 and 2, for anappropriate temperature range, simple equations relating resistance andtemperature may be given by

1/T=C ₆ +C ₇×(ln R).  Equation 5

ln R=C ₈ +C ₉ /T.  Equation 6

In terms of temperature T in degrees Celsius, the relationship may bewritten:

T=B/(ln R+A)−T _(k)  Equation 7

where A, B and T_(k) are empirical constants supplied by themanufacturer with each calorimeter.

The methods of the invention comprise irradiation of a dosimeter, suchas a calorimeter, and the systems of the invention comprise a source ofradiation effective to emit radiation. In a preferred embodiment, thesource of radiation is a high dose-rate radiation source such as anelectron radiation source.

The amount of radiation delivered to an item, that is, the radiationdose, is determined by several factors including the intensity of theradiation source, the duration of the irradiation, the distance betweenthe radiation source and the item irradiated, and the orientation of theitem with respect to the radiation pathway. Radiation dosage is commonlymeasured in Gray units, where 1 Gray is equivalent to the absorption of1 joule per kilogram. Thus 1,000 Gray units, or 1 kGy, represent theabsorption of 1000 joules per kg by an irradiated object. In a preferredembodiment of the invention, the electron radiation provides a dose ofbetween about 0.1 kGy to about 100 kGy. In a more preferred embodiment,the electron radiation provides a dose of between about 2 kGy to about70 kGy. In a most preferred embodiment, the electron radiation providesa dose of between about 3 kGy to about 40 kGy.

The invention provides a system for sterilization and a system forroutine dosimetry. The systems comprise a radiation source, a radiationpath along which radiation is emitted by the source, and a calorimeter.In preferred embodiments, the calorimeter is suitable for placement inthe radiation path effective to receive radiation from the radiationsource. In more preferred embodiments, the calorimeter comprises athermistor calorimeter.

In a preferred embodiment of the systems of the invention, the radiationsource is a high dose-rate radiation source such as an electron beamsource and the radiation emitted is an electron beam. In furtherpreferred embodiments of the system, the electron beam is effective toprovide a radiation dose of between about 0.1 kGy to about 100 kGy. Inmore preferred embodiments of the system, the electron beam is effectiveto provide a radiation dose of between about 2 kGy to about 70 kGy. Inmost preferred embodiments of the system, the electron beam is effectiveto provide a radiation dose of between about 3 kGy to about 40 kGy.

In a further embodiment, the systems further comprise a conveyoreffective to move the calorimeter. In preferred embodiments, theconveyor is effective to move the calorimeter quickly between the pointof irradiation and the point of testing. In most preferred embodiments,the conveyor follows a short, closed-loop route, such as a short ovalshaped route, effective to move the calorimeter quickly between thepoint of irradiation and the point of testing.

In a preferred embodiment, a method is provided for irradiating an itemby a radiation source, measuring heat evolved from irradiation,comprising the procedures of irradiating a calorimeter, measuring heatwith the calorimeter effective to provide a measurement of heat evolvedfrom the irradiating procedure, calculating a change in temperature, andcalculating a radiation dose using the calculated change in temperature.The calculated change in temperature may be made using the measurementof heat evolved from the irradiating procedure, among other factors.Such other factors may comprise other related measurements andparameters, including but not limited to an initial temperature,performance parameters of the calorimeter, and time after irradiation.

In preferred embodiments of the invention, the irradiation procedurecomprises irradiation of an item or a calorimeter where the item orcalorimeter is carried on a conveyor to contact a beam of radiation. Asillustrated in FIG. 3, the irradiation procedure comprises placement ofan object to be irradiated, which may be an item 50 or a calorimeter 52,on a conveyor 48 upon which the object is carried into the beam path 46where it is contacted by beam 44 and thereby irradiated.

In the most preferred embodiments of the methods, the irradiation of acalorimeter is performed in the same manner as the irradiation of anitem undergoing a radiation process. It is most preferred that theradiation source is a high dose-rate radiation source. In most preferredembodiments, the radiation is electron radiation. As illustrated in FIG.3 both item 50 and calorimeter 52 are carried by conveyor 48 throughbeam 44, along the same route, the route of conveyor 48. The irradiationof calorimeter 52 is thus performed in the same manner as theirradiation of item 50 undergoing the radiation process illustrated inFIG. 3. In such an embodiment, the calorimetry process is a part of theoverall irradiation process, which need not be stopped or altered toaccommodate the calorimeter. Such irradiation may be by any suitablemethod know in the art.

Any conveyor that is effective to carry an object to or within theradiation beam pathway is suitable for the practice of the invention.Thus, straight, angled, and curved conveyors may be used in the practiceof the invention. Conveyors may comprise chain conveyors, linkedpallets, conveyor belts, overhead suspension systems, railway systemsincluding monorail systems, and other conveyors known to the art. Motiveforce for transporting objects by conveyor may be of any suitable kind,including mechanical, hydraulic, electrical, or gravitational motiveforce. A preferred conveyor is a conveyor that follows a shortclosed-loop route. A short, closed-loop route such as an oval-shapedroute is preferred because it shortens the time needed to provide theobjects with a desired radiation dosage.

In addition, a short, closed-loop route such as an oval-shaped trackconveyor design is advantageous because such a design minimizes theamount of time between the loading an item for irradiation, and theultimate unloading of the item following irradiation. Benefits of thistype of system include the ability to place items in multiply-stackedlayers and the ability to place items in a favorable orientation withrespect to the radiation path for the most efficient use of the emittedradiation. Thus the use of a short, closed-loop route such as anoval-shaped track facilitates the practice of the present invention.

Another application of the invention is the use of robotic arms thatprovide for the testing of the calorimeter within the radiation vault ofa radiation processing facility that used a longer conveyor path. Suchrobotic testing would provide for the quick testing of the calorimeterafter irradiation.

Another advantage of the present invention is the simplicity of themethod. The routine dosimetry measurements may be collected by standardproduction operators with no special technical skills and with nospecial robotic arms within the radiation vault. Automated sensors onthe calorimeter, however, may simplify the process. For example, anautomated sensor that contacts a sensing lead upon entry to, and exitfrom, the radiation beam pathway provides automatic calorimetrymeasurements suitable for the practice of the invention. Thus, theroutine dosimetry measurements of the invention do not requireinordinate interruption of radiation processing, which is preferablyradiation sterilization, and are easy to obtain in the normal course ofsuch processing.

Prior methods required placement of dosimeter film, or other dosimeters,before and after designated items are irradiated in an electron beamfacility. In the practice of the present method, radiosensitive filmsare not used, and routine dosimetry is accomplished by the introductionof a calorimeter, such as a thermistor calorimeter, into the radiationprocessing facility at intervals. These intervals may be regularintervals, as taught in the method illustrated in FIG. 2 where thecalibration process is performed at least about once every hour.Alternatively, the intervals may be irregular, or random in time, or mayfollow a complicated pattern of time intervals. As taught in the methodsof the present invention, the calorimeter is processed in the samemanner as are the items being irradiated by the irradiation process.

Thermistor calorimetry data comprise resistance measurements. Suchmeasurements may be used in the determination of the radiation doseabsorbed per pass of the calorimeter through the radiation beam. Theradiation dose may be determined by calculating the dose using themeasured thermistor resistances. The raw resistance data obtained from athermistor calorimeter may be entered into a spreadsheet for use inperforming two calculations. It will be understood that any method ofperforming such calculations, whether by hand, hand calculator,spreadsheet, programmable computer, dedicated computational circuitry,or other methods, is suitable for the calculation of radiation dose perpass of the calorimeter through the radiation beam.

The first calculation comprises the translation of resistance data totemperature data, and then calculation of the change in temperatureresulting from irradiation (calculated by subtracting the calculatedtemperature measured before irradiation from the calculated temperaturemeasured and after the irradiation process). The translation ofresistance data into temperature data is performed by applying a known,calibrated relationship between resistance and temperature for thecalorimetry system being used (e.g., the resistance versus temperaturerelationship of a specific thermistor as in Equation 1 or Equation 2).

The second calculation is performed by translating the change intemperature into a radiation dose. This is done by means of a known,calibrated relationship (e.g., a National Dose Standards Laboratoryrelationship) between temperature change and radiation dose. Forcalibration, the radiation dose may be measured, for example, using analanine reference dosimeter. Alanine dosimeters may be obtained fromNational Physical Laboratory (NPL, Teddington, Middlesex, UK)).Similarly, radiation dose standards may be available from the NationalInstitute of Standards and Technology (NIST).

The relationship between temperature change and radiation dose may bedetermined by fitting a theoretical curve, such as a polynomial curve,to empirical data consisting of measured temperature change valuesresulting from known radiation dosages. Various methods, including useof a calculator, a programmable computer, a spreadsheet program runningon a computer, and other methods, are suitable for determining such arelationship. Such a calibration procedure is similar to the one used tocalibrate thermistor resistance as a function of temperature; or avendor may provide the R-T relationship. For example, a spreadsheetprogram may be used to perform regression analysis fitting oftemperature change data versus the radiation dose absorbed. By selectingan N^(th) degree polynomial that fits the data, and using the constantscalculated from it, the dose corresponding to any temperature changewithin the calibration range can be calculated. Commonly, a seconddegree polynomial will provide a good fit and enable the calculation ofa radiation dose from a temperature change. Thus, for example, Equation8 may be used for such calculations:

Y(dose kGy)=A+BX+CX ²  Equation 8

where Y is the calculated dose, X is the temperature change, and A, B,and C are constants determined by the calibration relationship.

The result of these calculations is a radiation dose per pass of thecalorimeter through the radiation beam, where a pass is one routineexposure of the calorimeter to the radiation beam. For example, wherethe conveyor carries the calorimeter in a closed loop route whichintersects the path of the radiation beam, a pass is one circuit by thecalorimeter along the route that results in a routine exposure to theradiation beam. Where the conveyor route is not a closed loop, a pass isa single complete passage of the calorimeter along the route.

Thus, the present invention provides methods and systems of routinedosimetry and calorimetry, and dosimetry methods that allow theaccomplishment of radiation sterilization and routine dosimetry by meansof calorimetry with fewer resources than previous methods and systems,with far superior quality and with far less compliance risk. The abilityto confirm and monitor the reliability of irradiation, such as electronbeam irradiation, is greatly enhanced since the variability of themeasuring tool is reduced. Sterilization is a heavily audited andcritical process. The ability to minimize the everpresent quality issuesrelated to dosimetry and calorimetry can significantly reduce compliancerisk and enhance the competitive advantage of electron beam radiationprocessing.

EXAMPLE 1

A short, oval-shaped track chain conveyor system was used to irradiatesamples (Surebeam®, Titan Scan Corporation, San Diego, Calif.). Theconveyor system and shielding design of the electron beam processaccommodates very small intervals between the time the items to beirradiated leave the load area and the time that the items arrive at theunload area after irradiation that is ideal for calorimeters. Typically,less than 5 minutes were needed to move product from the load areathrough the electron beam and to the unload area. The products wereconveyed via chain conveyors that move captive pallets and productfixtures into and out of the electron beam exposure area.

Radiation dose delivered by the electron beam was calibrated usingalanine reference dosimeters as standards. Alanine dosimeters wereobtained from National Physical Laboratory (NPL, Teddington, Middlesex,UK and were processed by NPL following exposure to measure the dosagereceived and to provide the radiation dosage calibration.

EXAMPLE 2

RISO National lab type calorimeters (RISO National Laboratories,Roskilde, Denmark, serial numbers #1095 and 1096) were validated for useas routine dosimeters for monitoring the performance of an electron beamprocess utilizing the short, oval-shaped track conveyor of Example 1(Surebeam®, Titan Scan Corporation, San Diego, Calif.). Each calorimeteris a stand-alone device and is ready to be used multiple times. Thecalorimeters were polystyrene type calorimeters with a Veco 32A180Thermistor (an NTC type thermistor), comprising a blown polystyrene foambox containing a polystyrene disc within the foam. Polystyrene hasexcellent radiation compatibility and insulation properties and is anideal material for the construction of a calorimeter.

In order to determine the radiation dosage received by a thermistorcalorimeter, two relationships must be known and calibrated for thecalorimeter: 1) the relationship between thermistor resistance andtemperature, and 2) the relationship between the radiation dosagereceived and the resulting temperature change.

The calibration relationship between the thermistor resistance andtemperature was provided by the vendor. It was confirmed and calibratedby imposing different temperatures on the thermistor by placing thethermistor in a temperature bath, monitoring the temperature andmeasuring the resistance at each of several temperatures.

The equation (see equation 7 above)

T(C)=B/(ln R+A)−T _(k)

describes the relationship between resistance of the thermistor and thetemperature of the thermistor. Fitted coefficients provided by thevendor for this equation were:

Thermistor A B T_(k) 1095 5.9708 4586.13 317 1094 5.9978 4590.72 317

The calorimeters were processed with doses ranging from 2 kGy to 70 kGytogether with alanine reference dosimeters by National PhysicalLaboratory (NPL, Teddington, Middlesex, UK). The alanine referencedosimeters were irradiated in a phantom obtained from Riso NationalLaboratory so that the alanine dosimeters and the thermistor receivedthe same dose. The resistance of the thermistor in the thermistorcalorimeter was measured just prior to and just after each irradiationto determine the temperature of the calorimeter and the change intemperature (ΔT) corresponding to each dose. The calorimeter was allowedto cool to room temperature between irradiations. The alanine referencedosimeters were sent back to NPL for evaluation. The relationshipbetween AT and the alanine reference dose was fitted with a second orderpolynomial.

The procedure for using the calorimeters included the following fourprocedures. First, to insure that the calorimeter was in thermalequilibrium before irradiation, a multimeter was used to measurethermistor resistance; calorimeters were not irradiated until initialresistance readings were stable at ambient temperature. The resistanceof #1095 at ambient temperature was 1900±50 kΩ and the resistance of#1096 at ambient temperature was 1870±50 kΩ. The initial resistancevalue was recorded in an Excel® (MicroSoft, Redmond, Wash.) spreadsheet.Second, the calorimeter was placed on a conveyor fixture and sent alongthe short, oval-shaped track into the electron beam at the desired speed(ranging from about 0.5 to about 8 feet per minute (fpm)). Third, at theunload station of the short, oval-shaped track, immediately followingirradiation of the calorimeter, the multimeter was again connected tothe thermistor and resistance was measured and entered in to thespreadsheet. Fourth, the calorimeter was allowed to cool (cooling wasassisted by a small fan) and was ready for further validation work whenthe measured resistance had fallen to ambient levels.

The calorimeter was placed on a pallet and processed in the same manneras other items being processed through the electron beam. The processingparameters were a conveyor speed of about 0.5 to 8 feet per minute (fpm)during exposure to the 10 MeV beam at a power of 6 kW, delivering a dosein the range of 5 to 100 kGy.

A Riso-type calorimeter can be processed at predefined intervals,minimally when electron beam parameters are changed. Typically, suchintervals can be one per lot of product, or once per every ten palletsof product processed through the electron beam. The interval used in themeasurements reported in this example was one hour. Such calorimeterprocessing at predefined intervals may be performed fewer than one timeper hour, and since the calorimeter is less than 1 foot in width, thisrepresents a loss of raw processing efficiency of ≦2% [≦(1 ft for acalorimeter)/(60 ft of processing per hour)].

Resistance data was collected directly from the calorimeter, immediatelybefore entry of the calorimeter into the electron beam and immediatelyafter irradiation of the calorimeter by the beam.

The RISO calorimeters #1095 and #1096 were irradiated while beingconveyed long a short, oval-shaped pathway through an electron beam(Surebeam®, Titan Scan Corporation, San Diego, Calif.) at severalconveyor speeds, resulting in the delivery of different dosages for thedifferent speeds. In one experiment for each calorimeter, doseattenuators were used for the fastest conveyor speed (8 feet per minute(fpm)) in order to reduce the dosage. The change in temperature wasmeasured for each different radiation dose. These results are shown inTable 1.

TABLE 1 Speed Dose Alanine (fpm) (kGy) R₀ (kΩ) R₁ (kΩ) T₀ T₁ ΔT 109536-177 8.0 2.2 1969 1924 21.31 21.89 0.58 (atten.) 36-178 8.0 4.4 20241727 20.62 24.61 3.99 36-179 5.77 6.6 2000 1612 20.92 26.38 5.46 36-1803.71 11 1904 1378 22.15 30.46 8031 36-181 2.14 20 1931 1109 21.79 36.2714.47 36-182 1.46 30 1912 866 22.04 43.13 21.09 36-183 1.25 35.3 1911765 22.06 46.67 24.61 36-184 1.11 40 1913 687 22.03 49.80 27.77 36-1850.89 50 1999 578 20.93 54.94 34.01 36-186 0.64 70 1961 383.3 21.41 67.7546.35 1096 36-177 8.0 2.2 1941 1901 21.33 21.85 0.52 (atten.) 36-178 8.04.4 1995 1703 20.65 24.62 3.98 36-179 5.77 6.6 1968 1579 20.99 26.565.57 36-180 3.71 11 1891 1360 21.98 30.44 8.46 36-181 2.14 20 1913 109721.69 36.18 14.49 36-182 1.46 30 1864 847 22.34 43.35 21.01 36-183 1.2535.3 1882 757 22.10 46.56 24.46 36-184 1.11 40 1858 669 22.42 50.1527.73 36-185 0.89 50 1975 570 20.90 54.92 34.02 36-186 0.64 70 1943380.6 21.30 67.50 46.19

Resistance measurements taken before and after exposure of thecalorimeter to the electron beam (as described above) were used tocalculate the difference in temperature ΔT induced by the radiationexposure. The relationship between temperature and radiation dose(reported by NPL from the alanine dosimeters) was defined by performinga regression of the ΔT data and the absorbed dose (as determined by thealanine dosimeters irradiated at the same time as the calorimeters; theradiation received by the alanine dosimeters was measured by NPL so thatthe doses used for the calibration were traceable to a nationalstandards dosimetry laboratory). Several polynomials (a linear equation,and 2^(nd), 3^(rd), 4^(th), and 5^(th) degree polynomials) were fittedto the temperature and dosage data. The 2^(nd) degree polynomial wasfound to have the lowest overall percentage error. Second degreepolynomials of the general form

Y(dose kGy)=A+B(X)+C(X)²

were found to give the best fitting to the data with the followingconstants:

Calorimeter A B C 1095 0.0521 1.2621 0.0058 1096 0.0065 1.2631 0.0059.

After initial and final resistances were measured, the resistance valueswere entered into an Excel® spreadsheet and used to calculate theinitial and final temperatures, and then the ΔT. From the ΔT value,using the equation and constants above, the radiation dose wascalculated.

Using these constants, the following calculated ΔT values and radiationdosages are shown in Table 1 and Table 2 along with the dosages measuredby the alanine reference dosimeters exposed at the same time. Theabsolute percentage errors are also given in Table 2.

TABLE 2 Calculated Dosage Absolute Alanine ΔT (C) Absolute 2^(nd) Degree% Reference calorimeters Dosage Polynomial Error ± # 1095 1096 (NPL)1095 1096 1095 1096 36-177 0.58 0.52 0.353 36-178 3.99 3.98 5.28 5.185.13 1.9 2.9 36-179 5.46 5.57 7.26 7.12 7.23 2.0 0.5 36-180 8.31 8.4611.14 10.94 11.11 1.8 0.2 36-181 14.47 14.49 20.1 19.53 19.55 2.8 2.736-182 21.09 21.01 28.8 29.25 29.15 1.6 1.2 36-183 24.61 24.46 34.434.63 34.43 0.7 0.1 36-184 27.77 27.73 39.6 39.57 39.57 0.1 0.1 36-18534.01 34.02 49.6 49.68 49.81 0.2 .04 36-186 46.35 46.19 71.1 71.01 70.940.1 0.2

In order to be meaningful, a dosage measurement should also beaccompanied by an estimate of the uncertainty, as discussed in ASTM1631. The measurement uncertainty was termed Total_(ff) Uncertainty, andexpressed as a percent. Total_(ff) Uncertainty % was calculated from twounderlying uncertainties, the uncertainty in the temperature measurement(due to the uncertainty in the correlation between thermistor resistanceand temperature, and the component measurements) and the uncertainty inthe radiation dosage measurement (due to the uncertainty in thecorrelation between temperature change and radiation dosage, and thecomponent measurements). The uncertainty in the measurement oftemperature from thermistor resistance was termed Total_(final)Uncertainty, and was expressed as a percentage. The uncertainty in thedetermination of dosage from the temperature change was termedDose_(max) Uncertainty and was determined by differences between thedose estimates calculated from the T-D data fitted to a 2nd degreepolynomial compared to the absorbed dose measured using the NPL alaninedosimeters. The Total_(ff) Uncertainty % was calculated by taking thesquare root of the sum of the squares of the underlying uncertaintiesTotal_(final) Uncertainty % and Dose_(max) Uncertainty. These values aregiven in the following Table 3.

TABLE 3 Total_(final) Dose_(max) Total_(ff) Uncertainty % Uncertainty %Uncertainty % 1095 1096 1095 1096 1095 1096 ± 2.58 ±2.42 ±2.84 ±2.90±4.12 ±4.10

The component uncertainties for the temperature measurement(Total_(final) Uncertainty %) and dosage measurement (Dose_(max)Uncertainty %) shown in Table 2 thus may be combined as the Total_(ff)Uncertainty % and used to define the acceptance criteria for dosimetricrelease by calorimetry during routine production. The evaluation showedthat the total uncertainty in this example was 4.1%.

Thus, the RISO calorimeters #1095 and #1096 were calibrated for use asroutine dosimeters for monitoring the performance of an electron beamsterilization process utilizing the short, oval-shaped track conveyor ofExample 1. Thermistor resistance measurements were used to determinetemperature change and the ΔT was used to determine radiation dosageusing the relationships and polynomial constants determined from fittingthe measured ΔT and dosage calibration data. Resistance data from thethermistor calorimeter was recorded directly before entry of thecalorimeter into the electron beam, and immediately after irradiation ofthe calorimeter. The raw resistance data was entered into a spreadsheetto accomplish two calculations. The first calculation translated theresistance data into temperature, and then the change in temperaturefrom before to after the irradiation. The first calculation wasaccomplished by means of the relationship between resistance andtemperature for the specific thermistor used in the measurement. Thesecond calculation translated the change in temperature into a radiationdose. This translation of temperature change into dose was done usingthe relationship between the change in temperature and the radiationdosage obtained using alanine dosimeters obtained from, and evaluatedby, NPL.

EXAMPLE 3

The method and system for routine dosimetry herein disclosed is suitablefor use with other radiation processes. It has been integrated into thevalidated SureTrack electron beam process control and record system(Titan Scan Systems, San Diego, Calif.) to accelerate production withfewer resources and far superior quality.

The SureTrack system prompts the operator to process a calorimeter atthe appropriate frequency and automatically calculates the dose receivebased on the resistance differential of the calorimeter. The dose valuesare then automatically recorded in the process report, providing an easymethod to verify and document that the product received the proper dose.

Through the use of thermistor calorimetry as routine dosimeters in anelectron beam sterilization process, the radiation dosage deliveredduring each processing run may be measured and recorded for processcontrol, verification, and documentation. In such a report, such processdetails as identifying serial numbers of the calorimeters and otherequipment used may be included, and details of the electron beam,including the process limits (low and high) for acceptable performanceof the electron beam. For example, the beam current (typically in mA),power (typically kW), beam repetition rate (typically as pulses persecond), scan magnet current (typically mA), scan magnet frequency(typically Hz), process rate (typically fpm), and the dosage delivered(typically kGy) are some important process parameters that may beincluded in a routine process report as part of the routine procedure ina radiation process.

The following is an example of such a process report used in the routineprocess, an E-Beam History Report.

Calorimeter ID Resistance at Load Resistance at Unload Δtime N1082-00101376 Ω 547 Ω 12 min N1082-0004 2407 Ω 950 Ω 11 min Low High Limit LimitFirst Pallet Last Pallet Ave. Beam Current (mA) 362 406 380 380 KlystronRF Ave. Power (kW) 3.85 4.14 4.00 4.07 Modulator Rep Rate (pulses/s) 190210 200 200 Scan Magnet Current (mA) 50.2 55.4 52.0 52.0 Scan MagnetFrequency (Hz) 6.00 8.00 6.95 7.00 Process Rate (items/hour) 1.243 1.2571.252 1.253 Dose (kGy) 20.0 50.00 36.66 33.67

The invention thus provides a fully integrated routine dosimetry systemthat is easy to use, requires minimal resources, provides completedocumentation and minimizes quality and compliance issues.

It will be apparent from the foregoing that, while particular forms ofthe invention have been illustrated and described, various modificationscan be made without departing from the spirit and scope of theinvention. Moreover, those skilled in the art will recognize thatfeatures shown in one embodiment may be utilized in other embodiments.

What is claimed is:
 1. A sterilization system for a medical product,comprising: a controlled radiation source effective to provide a dose ofradiation along a radiation path; a calorimeter controller configured toperform the steps of a radiation dosimetry control method; and acalorimeter; wherein the dosimetry control method comprises causing theradiation source to provide a dose of radiation and measuring thetemperature of the calorimeter at an interval determined by thecalorimeter controller based on the dose of radiation provided.
 2. Thesterilization system for a medical product of claim 1, furthercomprising a conveyor effective to convey said thermistor calorimeterthrough said radiation path within a short time.
 3. The sterilizationsystem for a medical product of claim 2, wherein said conveyor iseffective to convey the thermistor calorimeter along a short,closed-loop route at a rate effective to convey the thermistorcalorimeter from a starting position to an ending position within ashort time.
 4. The sterilization system for a medical product of claim3, wherein said controlled radiation source comprises a high dose-ratesource.
 5. The sterilization system for a medical product of claim 4,wherein said high dose-rate radiation source is an electron radiationsource effective to provide a dose of electron beam radiation.
 6. Asystem for routine dosimetry for quality control of a radiation process,comprising: a controlled radiation source effective to provide a dose ofradiation along a radiation path; a calorimeter controller configured toperform the steps of a radiation dosimetry control method; and acalorimeter; wherein the dosimetry control method comprises causing theradiation source to provide a dose of radiation and measuring thetemperature of the calorimeter at an interval determined by thecalorimeter controller based on the dose of radiation provided.
 7. Thesystem of claim 6, further comprising a conveyor effective to conveysaid thermistor calorimeter through said radiation path within a shorttime.
 8. The system of claim 7, wherein said conveyor is effective toconvey the thermistor calorimeter along a short, closed-loop route at arate effective to convey the thermistor calorimeter from a startingposition to an ending position within a short time.
 9. The system ofclaim 6, wherein said controlled radiation source comprises a highdose-rate radiation source.
 10. The system of claim 9, wherein said highdose-rate radiation source is an electron radiation source effective toprovide a dose of electron beam radiation.
 11. The system of claim 10,wherein the dose of electron beam radiation is between about 0.1 kGy toabout 100 kGy.
 12. The system of claim 10, wherein the dose of electronbeam radiation is between about 2 kGy to about 70 kGy.
 13. The system ofclaim 10, wherein the dose of electron beam radiation is between about 3kGy to about 40 kGy.
 14. The system of claim 6, wherein said calorimeteris a thermistor calorimeter, and further comprising a movable roboticarm having a resistance measuring device effective to contact thethermistor calorimeter and to obtain a resistance measurement therefrom.15. The system of claim 6, wherein the steps that said calorimetercontroller is configured to perform comprise the steps of accepting foruse only validated calorimeters, determining whether the targetradiation dose from said controlled radiation source has been changed,maintaining said interval constant if said target radiation dose has notbeen changed, prompting a routine calorimeter dosimetry if said targetradiation dose has been changed, and managing the printing of a processreport.
 16. The system for routine dosimetry of claim 6, wherein saidcontroller comprises an automatic controller.
 17. The system of routinedosimetry of claim 16, wherein said automatic controller comprises acomputer-controlled automatic controller.